As interest in environmental problems has progressively increased in recent years, a study for fuel cell has been actively performed as the alternative to a gasoline engine and fossil fuel. To make the best use of the fuel cell, the various requirements for producing and/or supplying hydrogen used as raw material should be prepared.
Hydrogen is the lightest gas and may be exploded easily in the air. Due to the above properties of hydrogen, it is extremely difficult to handle and store hydrogen. Accordingly, a current level of technique is to supply hydrogen by merely using a hydrogen storage tank with a high capacity, the above technique has a drawback that it is difficult to solve completely the aforementioned problems by means of such hydrogen storage tank.
In addition, there is a problem that an initial facility investment expense is immensely required for developing the hydrogen storage tank with a high capacity and providing a hydrogen storage facility with a high capacity.
Accordingly, the method by which a volume and weight of the hydrogen generating equipment are reduced may be very preferable in that hydrogen can be used as clean energy without paying an immense facility investment expense.
In addition, to supply directly hydrogen generated in the miniaturized hydrogen generating equipment to the fuel cell, a content of carbon monoxide (CO) which impedes an activity of a cathode electrode of the fuel cell should be minimized.
That is, carbon monoxide in hydrogen-rich gas generated by a reforming reaction between steam and hydrocarbon fuel poisons platinum used as an electrode material of a proton exchange membrane fuel cell to lower rapidly a performance of the fuel cell.
The permissible concentration level of carbon monoxide which is suitable for preventing a poison of platinum caused by carbon monoxide is approximately 50 ppm or less. To generate carbon monoxide with a suitable concentration, a water-gas shift reaction process and a preferential CO oxidation reaction process are performed for hydrogen-rich gas.
After performing the water-gas shift reaction process, a concentration of carbon monoxide should be decreased to 1% or less, and after the preferential CO oxidation reaction process, a concentration of carbon monoxide should be decreased to 50 ppm or less.
A commercially available water-gas shift reaction is performed in two stages, that is, a high temperature water-gas shift reaction stage and a low temperature water-gas shift reaction stage, and may be represented by the following reaction formula:CO+H2O═CO2+H2 ΔH=−41.1 kJ/mol  [Reaction formula]
Fe2O3/Cr2O3 catalyst is used in the high temperature water-gas shift reaction performed at a temperature of 350 to 450° C., and the low temperature water-gas shift reaction in which Cu/ZnO is used as catalyst is performed at a temperature of 200 to 300° C.
Accordingly, the system for performing the high temperature water-gas shift reaction and the system for performing the low temperature water-gas shift reaction should be provided separately. A complicated heat-exchange system equipped with sophisticated temperature controlling devices is required for maintaining the appropriate temperature in the high and low temperature water-gas shift reaction system.
The two reaction systems mentioned above increase the overall size of the system. Moreover, the difficulty of controlling temperature leads to lower overall system stability.
In addition, the water-gas shift reactor system generally employs a fixed-bed reactor. This fixed-bed reactor can not remove effectively and completely heat generated when the water-gas shift reaction which is the exothermic reaction is performed, and so the fixed-bed reactor has a drawback that the uniform temperature distribution is not obtained. Due to the above drawback, the fixed-bed reactor has the problems that the expected life span of the catalyst is reduced and a shift ratio is decreased. Accordingly, the system that is capable of removing completely heat generated when a reaction is performed is required.